Description of the Background Art
Recent computer systems include a large-sized processing circuit for calculating three-dimensional coordinates, thereby increasing the number of polygons that can be displayed in one frame.
A technology for reducing a load of a three-dimensional image processing process in one frame in order to perform another process in one frame is disclosed in Japanese Patent Laid-Open Publication No. 2003-228725 (hereinafter referred to as a patent document). In the patent document, when many polygon models similar in shape formed of a plurality of polygons have to be simultaneously displayed, as is the case of representing bushes, a polygon model having a typical shape is rendered in advance and its image is prepared, and that image is pasted on a polygon, thereby displaying each bush in a simplified manner. Thus, the number of polygons per frame is reduced, thereby increasing a frame rate.
In recent years, in the above-described computer system and a non-portable video game machine an advanced three-dimensional image processing circuit using more realistic game images with higher definition, that is, using a large number of polygons, has been adopted, to the extent that it has a large share of the market. For example, video game machines capable of processing twenty million polygons per frame have been available.
An advanced three-dimensional image processing circuit is also desirable for incorporation in portable game machines. However, with an increase in the circuit size of such an advanced three-dimensional image processing circuit, problems occur, such as an increase in the area of a semiconductor chip, an increase in the cost required for research and development of the image processing circuit, and an increase in heat, leading to a huge increase in the size of the housing of the game machine. Therefore, in the portable game machines, which are desirably provided in small sizes and at low costs, in the interest of portability and availability, such an advanced three-dimensional image processing circuit is more problematic than preferable.
As such, the portable game machines has incorporated therein a three-dimensional image processing circuit that is smaller than that in the computer system described above. In such a small-sized three-dimensional image processing circuit, however, the number of polygons processable per frame is relatively small (for example, several thousands polygons processable per frame). Therefore, what can be displayed is limited to non-realistic three-dimensional images.
Also, the technology disclosed in the above-described patent document is effective when rendering a scene in which many polygon modes similar in shape to one another are present, but is not effective in reducing the number of polygons to be processed when no polygon models similar in shape to one another are present. Moreover, in the technology disclosed in the above-described patent document, many polygon models similar in shape to one another are approximately displayed by using the image of a typical polygon model. Therefore, although the number of polygons to be processed may be reduced, scenes cannot be displayed with a high degree of fidelity.
Furthermore, as is the case of the conventional technology, increasing the frame rate through a simplified polygon process is effective when, for example, a game image including a subject moving at high speed in a game space is displayed, as in a race game. However, a liquid crystal display generally adopted to the portable game machines has a response speed slower than that of a CRT display or the like. Therefore, increasing the frame rate may not be as effective, depending on the liquid crystal display. Also, in a game in which subjects do not move much or move as rapidly as they do in a simulation, it might be desirable to display fine images with many polygons, even at the expense of the frame rate.